Motor controllers are used with motors to provide variable and controllable speed for various applications. The same motor controller may be used for different applications in the same system; for example, a motor controller in an aircraft may be used to start a main engine and to drive an environmental control system.
To minimize the size and weight of the motor used in a given application, it is common to drive the motor at high rotational speeds. This in turn requires the motor controller to operate at relatively high frequencies. To do this, the motor controller may include a motor drive having switches, such as switching transistors, with switching frequencies that can handle the rotational speed of the motor. However, it is also necessary to prevent excessive radiated electromagnetic interference (EMI) emissions due to the high frequency operation of the motor drive. Although shielding of input and/or output lines of the motor drive is often used to control EMI emissions, there are weight-sensitive applications, such as aircraft applications, the extra weight and heat retention caused by the shields would be unacceptable.
Common mode filtering is therefore considered a more desirable way to control radiation emissions from the motor drive. Common mode filtering controls the common mode voltage at the input and/or output feeders of the motor drive. Common mode voltages normally occur at the output lines of the motor drive. As is known in the art, inverter switches, such as insulated gate bipolar transistors (IGBTs), operate at a chop frequency typically ranging in the thousands of kHz and at amplitudes of hundreds of volts. The common mode elements of the chop frequency and its harmonics must be attenuated to a tiny fraction of the original chop voltage (e.g., on the order of millivolts) to reduce EMI emissions to required levels.
Typically, the input feeders of the motor drive are grounded to a system ground (e.g., an airframe) and the output feeders (i.e., the lines going to the motor) float and stay electrically isolated from the system ground. Common mode filtering is typically realized in such a system by adding a common mode filter, such as a low pass filter, to the output feeders of the motor drive.
The load for the motor drive is typically an ungrounded motor, making it easy to design the common mode filter to attenuate the common mode voltages so that they meet emission requirements. That is, none of the motor windings in the motor are connected to ground. However, for motor drive operating frequencies below the emission requirement limits, it may be difficult to keep the common mode voltage under control because the common mode filter itself has a resonant frequency. If the common mode filter is excited at its resonant frequency, it could generate extremely high common mode voltages on the motor and its associated feeder lines because there is no common mode path to ground. Because the motor drive operates over a wide frequency range, there are opportunities for the motor drive frequency to pass through the common mode filter resonant frequency and excite the common mode filter, thereby creating a risk of motor damage due to the resulting high voltage response.
Harmonics of the voltage output of the motor drive may also create undesirable levels of common mode voltage. Although a harmonic (e.g., a third harmonic) of the fundamental motor drive output voltage may be added to the fundamental motor drive output voltage itself to deliver more of the fundamental voltage to the motor (and therefore make more power available to the motor), the common mode filter will react to the harmonic. This reaction must therefore be addressed in the motor controller design to prevent the common mode filter from generating the high voltage response to the harmonic.
There is a desire for a system that provides tighter control over the common mode voltage in a motor controller.